Can a character move trough a square with a friendly character as if he isn't there?

Can he move trough an intersection between :a) 2 friendly characters?
b) a friendly character and blocking terrain?
C) a friendly character and hindering terrain?

If the terrain is clear and the moving character is able to complete his movement and end in a legal square, then he can move through a friendly.

Same goes for 2 friendlies

Same for a friendly and blocking or hindering.

Choose the past of least resistance when moving. I'm sure someone here will give you a more comprehensive answer/explanation on movement if you require it.

Long answer short, you can move through squares occupied by friendly characters. If you have flying, leap/climb or phasing you can move through squares with hindering terrain and over elevated or blocking terrain. If you don't have those powers, you will probably stop on hindering terrain (even if it's already occupied by a character)